Another class of problem concerned with the finite difference formulae in
the numerical analysis is to find them in case of unequal subintervals. This
situation specially arises in the mathematical modelling when the function has
been obtained experimentally and the independent variable is not under the control
of the experimenter, thus one need to find the finite difference formulae for the
observational data. To tackle this situation, by introducing generalized
Vandermonde determinant, Li [2] presented general explicit difference formulae
with arbitrary order accuracy for approximating first and higher order derivatives,
which can be used for both equally and unequally spaced data. However, we use
the available Lagrange’s interpolation formula to obtain the finite difference
formulae for unequally spaced subintervals.